Plain paper still is a favored recording medium for storing and transferring human readable information, but the emergence of electronic document processing systems has made it evident that the functional utility of plain paper and other types of hardcopy documents could be enhanced significantly if the human readable information they normally convey was supplemented by writing appropriate machine readable digital data on them. This machine readable data would enable the hardcopy document to actively interact with such a document processing system in a variety of different ways when the document is scanned into the system by an ordinary input scanner. See, for example, the copending and commonly assigned U.S. patent applications of Frank Zdybel, Jr. et al. and Walter A. L. Johnson et al., which were filed May 30, 1990 on "Hardcopy Lossless Data Storage and Communications for Electronic Document Processing Systems" and on "Form and System Utilizing Encoded Indications for Form Field Processing", respectively. As a general rule, digital data is recorded by writing two dimensional marks on a recording medium in accordance with a pattern which encodes the data either by the presence or absence of marks at a sequence of spatial locations or by the presence or absence of mark related transitions at such locations. Ordinary magnetic and optical digital data recording conform to this style of encoding. Furthermore, the bar-like codes which have been proposed previously for recording digital data on paper also conform to the above-described encoding style. See U.S. Pat. No. 4,692,603 on "Optical Reader for Printed Bit-Encoded Data and Method of Reading Same," U.S. Pat. No. 4,728,783 and U.S. Pat. No. 4,754,127 on "Method and Apparatus for Transforming Digitally Encoded Data into Printed Data Strips," and U.S. Pat. No. 4,782,221 on "Printed Data Strip Including Bit-Encoded Information and Scanner Contrast.
Considering the aforementioned bar-like codes in some additional detail, it will be seen that their visual appearance is highly variable because it is data dependent, so they tend to have a mottled appearance. This mottling is a readily discernible departure from the clean, crisp appearance of high quality printed documents, so it may be aesthetically objectionable to some observers. Furthermore, another drawback of these bar-like codes is the overhead that they contemplate. In particular, as taught by the above-identified patents, this overhead includes the registration marks which are provided for preserving the data clock, as well as the header information which is provided for describing the organization of the encoded data, such as the number of bits encoded along a given line of code. It, therefore, will be evident that there is an urgent need for relatively efficient, visually improved codes for recording digital data on plain paper and other types of hardcopy recording media, especially for applications in which such machine readable data is to be recorded in visual juxtaposition with human readable information. Furthermore, it will be appreciated that there is a need for efficient and reliable techniques for recovering digital data from such codes. Moreover, inasmuch as images carried by hardcopy documents often are replicated, such as by photocopying and by facsimile reproduction, it will be apparent that it would be very beneficial to have data encoding and decoding techniques that can tolerate a significant amount of image distortion.